1. Field of the Invention
The present invention relates to an ink supply device for an inkjet printer and an inkjet printer.
2. Discussion of the Background
An inkjet printer is an apparatus which forms images of information such as characters, graphics, patterns, and photographs on a print surface by ejecting fine particles of ink from a plurality of nozzles, which are formed in a print head, to deposit the ink on a print medium while moving the print head relative to the print medium. Because of the structure, as the inkjet printer has remained in the non-operating state for a long period of time, ink residual thickens around nozzle peripheries of the print head, thus not allowing the ink to be ejected properly. For solving this problem, there is an inkjet printer which includes a suction route for performing forcible vacuum suction of ink remaining in the print head in the state that the nozzle face of the print head is capped by a rubber cap, for example, on start-up (for example, see JP-A-2007-216535). In this way, thickening ink is sucked and removed and, at the same time, new ink is supplied into the print head, thereby reestablishing the print head to a state that ink can be ejected properly.
Further, in the inkjet printer, since ink is consumed according to the ejection of the ink, a carriage of the print head or a printer body is provided with an ink tank (ink cartridge) having a volume based on the intended use. In case of a large-sized inkjet printer for printing large commercial advertisements, banners, and the like, a large amount of ink is consumed in a relatively short time. In such a large-sized inkjet printer, therefore, a large volumetric ink tank (main tank) is generally provided in the printer body, and the ink tank and the print head are connected through tubes or the like so as to supply ink from the ink tank to the print head.
However, as the inner pressure of the print head becomes higher than the normal atmospheric pressure, a problem that ink is pushed out of nozzles to drip onto a print medium, i.e. a dripping problem occurs. To solve this problem, there is known an ink supply device of a “negative pressure producing type” which includes a sub-tank of a smaller volume disposed at the supply passage connecting main tank and print head, in which the print head is made into a state of slight negative pressure by reducing the pressure of the sub-tank (see, for example, JP-A-2004-284207 and JP-A-2006-62330).
The ink supply device of the aforementioned type is controlled such that a predetermined amount of ink is stored in the ink chamber of the sub-tank according to the amount of ink ejected from the nozzles to prevent interruption of ink supply to the print head. As one example of the control, it is controlled to supply ink from the main tank to the sub-tank when the ink level in the sub-tank is detected to be lowered to a predetermined lower limit. As a regular means of detecting the ink level, a structure has been disclosed in which a float provided with a magnet is vertically movably placed to float on ink and a sensor for detecting magnetism from the magnet is disposed at a predetermined level (for example, JP-A-2001-141547).
In addition, a pressure controller connected to the ink chamber of the sub-tank is usually provided in the ink supply device to adjust the pressure within the ink chamber. For example, it reduces the inner pressure of the ink chamber by sucking air from the inside of the ink chamber to set the print head at a state of slight negative pressure. The pressure adjustment becomes difficult if ink flows into the connector channel connecting the sub-tank and the pressure controller, and a dripping problem is likely to occur. Thereby, when, for example, the ink level excesses an upper limit in the ink chamber, it is controlled to prevent ink from the main tank from being supplied to the sub-tank.
As aforementioned, the nozzle face of the print head is capped for achieving the suction of ink, however, if for example there is a displacement between the nozzle face and the rubber cap, suction force may be reduced because air enters through a space between the nozzle face and the rubber cap. In this case, ink residual within the print head is sucked and removed, but new ink is hardly supplied to the print head so that the print head tends to be in a state containing (air) bubbles and consequently being not filled with ink. If the ink is ejected from the nozzles in this state, there is a problem that it is difficult to achieve stable ejection of ink because defective ejection occurs in which air bubbles not ink are ejected from the nozzles.
To precisely detect the ink level by the float which floats on the ink to move straight in the vertical direction according to the changes of the ink level in the sub-tank, it is required to use a large float, for example, corresponding to the volume of the ink chamber. If such a large float is used, it is possible to detect precisely the ink level in the sub-tank, but there is a problem of limiting the volume for ink storage.
Besides, the float used for detecting the ink level in the sub-tank may stick to the internal wall of the ink chamber, which makes it impossible to correctly detect the ink level. In this case, it is difficult to control the ink supply which is based on the correct detection of the ink level, and the ink is undesirably supplied to excess the predetermined upper limit. There is a problem that, when the ink is supplied over the predetermined upper limit to the ink chamber, the excessively supplied ink may flow to the side of the pressure controller (such flow of ink is referred to thereafter as “backflow”), making difficult the pressure adjustment within the ink chamber.